Problem with Alto 660cc

I have Japanese alto 660 cc since Oct 2007. It had been giving me fairly good fuel consumption with an average of 14 to 16 kmpl. However, over the last few weeks it has dropped down to 11 to 12 kmpl. Besides, the pick drastically reduces when I turn on the AC and even stalls when idling.
I have replaced the air filter but there is no improvement. Can anyone suggest me a solution? I would also like to know any reasonable workshop in Islamabad / Rwp from where computerized tuning can be done.

The car has done 45,000 kms and I have already got the Sparkplugs checked and cleaned. And yes, when the engine stalls I do get the tapped sound.
I did consult three different mechanics, one offered to completely open up the throttle and clean up everything including different sensors. I am not sure if that should be done for an EFI car. Any suggestion on it?
Regarding fuel filter I have been told that it is integrated with the fuel pump in the main tank and cannot be cleaned, complete pump needs to be replaced.
Last suggestion was to have the car tuned up through computerized scanner and not to open up anything at all. Where can I get the car checked through a computer in Islamabad, need a reliable workshop.

@Panther
computerized tuning wont do any thing gud if its due to dirty throttle body if your mechinic is a good one get it washed completely it wont harm your vehicle b/c i did the same thing with my city but that was due to fuel comsumption my car became jin few months back after getting the throttle body completly washed its now fine Allhamdullillah ...
@Junaid_A
you must have wrote it completly sir jee anyways i m sorry for that

I have Japanese alto 660 cc since Oct 2007. It had been giving me fairly good fuel consumption with an average of 14 to 16 kmpl. However, over the last few weeks it has dropped down to 11 to 12 kmpl. Besides, the pick drastically reduces when I turn on the AC and even stalls when idling.
I have replaced the air filter but there is no improvement. Can anyone suggest me a solution? I would also like to know any reasonable workshop in Islamabad / Rwp from where computerized tuning can be done.

if the car is EFI ho sakta hai that the injectors are clogged and need servicing via ultra sonic cleaning!

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